Generally, mid and low power LED-based lighting devices comprise a white cup, one or more LED chips for emitting blue light and a phosphorous light transmitting dielectric for converting the wavelength of the emitted blue light. The emitted blue light reaching the wavelength converting phosphor will usually result in yellow light (depending on the properties of the wavelength converting phosphor). Although some of the light output from the LED-based lighting device may be yellow, the output light will have a different spectral composition due to the fact that various light rays emitted at different angles from the LED chips pass an unequal amount of phosphor.
Hence, there will be a spatially depending difference in spectral composition in the light emitted from the LED-based lighting device, which is called color over position (COP).
Light from a LED-based lighting device exhibiting such COP will, after passing a collimator, result in an angular difference in spectral composition in the far field, called color over angle (COA).
A problem with using LED-based lighting devices showing COA in spot applications is that severe coloration in the fringe of a pattern can occur, especially for narrow light beams.